The technical field of this invention is magnetic resonance imaging (hereinafter referred to as "MRI") and, in particular, MRI systems which employ pulsed gradient coils to provide magnetic field gradients.
MRI systems measure the density distribution or relaxation time distribution of nuclear spins in objects, which can be inanimate or living. Because MRI systems offer the potential for images without the invasiveness of surgery or X-ray photography, MRI holds great promise for advances in medical research and diagnosis.
Magnetic resonance imaging ("MRI") is based on the phenomenon of nuclear magnetic resonance ("NMR"). When an object is placed in a magnetic field, the field causes the spin vectors of certain types of nuclei (e.g. .sup.1 H, .sup.13 C, .sup.31 P and .sup.23 Na) to orient themselves with respect to the field. The nuclear vectors, when supplied with the right amount of energy, will reorient themselves in the field and emit or absorb energy in the process. The energy needed to perturb the nuclear spin vectors is in the radio frequency range, and the specific frequency depends on the strength of the magnetic field experienced by the nuclei. In NMR analysis, the sample is placed in a large, uniform, static magnetic field. The sample is perturbed by a pulse of radio frequency energy, and the frequency response to this perturbation is recorded. A measure of intensity as a function of resonance frequency or magnetic field at the nucleus is obtained.
Imaging techniques carry the analysis one step further by using magnetic field gradients in addition to the base uniform magnetic field. Since the resonance frequency of the nuclei depends on the magnetic field strength, field gradients provide spatial encoding. MRI devices correlate signal intensity at a given frequency with sample concentration at a given location. This provides a map or image of the object which is based on intensity variation due to concentration or relaxation time differences. The essential field gradients are produced with a set of gradient coils. These coils are often referred to as "pulsed gradient coils" because they are energized by pulses which grade the background field in two or more orthogonal directions.
Imaging the entire body of a patient typically requires a steady, high homogeneity, background field of at least 0.5 tesla and highly linear gradients in the range of 1 gauss/cm with rise and fall times as short as possible, typically on the order of a few milliseconds. An axial gradient (i.e. in the "Z" direction) is typically produced by solenoid coils while radial gradients (which define "X" and "Y" coordinates) are formed by saddle-shaped coils. For a further discussion of MRI systems, see Bradley et al., "Physical Principles of Nuclear Magnetic Resonance" in Vol. 2 Modern Neuroradiology: Advanced Imaging Techniques. pp 15-61 (Newton and Potts, Eds. 1983) herein incorporated by reference.
Regardless of the way in which the background field is produced, be it by conventional resistive, permanent or superconducting magnet systems, the changing magnetic fields from the pulsed gradient coils will induce eddy currents in any nearby conducting media. These eddy currents have an adverse effect on both the spatial and temporal quality of the desired gradient fields. That is, the eddy currents will perturb the field from the desired level and quality in both space and time. In order to minimize the effects of these eddy currents, the gradient coils must be operated at an increased power level (i.e. overdriven in order to attain the desired field level in the desired time) or a longer rise time must be used to reduce the magnitude and effect of the eddy currents (thereby increasing the total scan time). However, the harmonic content of the gradient field will still change as the eddy currents decay (i.e. the gradient linearity will change during the pulse).
Furthermore, if the system generating the background field is superconducting, additional undesirable effects occur. In such a system, surrounding conducting media include cryogenic components (e.g. the radiation shield and liquid helium vessel), and Joule heating due to the induced eddy currents will cause an increased boil-off of cryogens. A more subtle effect can also occur as a result of the properties of the superconducting materials from which the conductor is fabricated. Such materials, when exposed to time varying magnetic fields exhibit electrically resistive behavior. This manifests itself as a decay in current in the superconducting coil or coils. A decay in the current of the background field coil degrades the temporal quality of the background field; while a current decay in the superconducting shim coils degrades the field uniformity.
Even in MRI systems that use resistive or permanent magnets to establish the background field, the gradient coils will induce eddy currents in nearby conducting media that will adversely affect the temporal and spatial field quality.
Existing solutions to the eddy current problem are less than satisfactory. Use of non-conductive bore tubes reduces the magnitude of the perturbations of the pulsed gradient field but creates greater cryogen boiloff and greater interaction of the pulsed gradient field with the background field coil systems. Heavy passive shields (typically thick-walled, copper cylinders) placed between the room temperature bore tube and the liquid helium vessel protect the superconducting system but result in additional heat loads at an intermediate temperature (e.g. liquid nitrogen temperature) as well as behavior which is unpredictable and dependent on pulse sequence. Overdriving the pulsed gradient system can produce the desired rate of rise of pulsed gradient field but does not resolve the problem of the spatial and temporal quality of this field, and the amount of "overdrive" necessary again is dependent on the pulse sequence employed.
There exists a need for more efficient, higher resolution, MRI systems. Magnetic resonance imaging devices which are capable of suppressing eddy currents and/or improving the quality of the field in both space and time would satisfy substantial needs in the art.